The present invention relates to a mobile communication system in which base station apparatuses based on the IS-95 scheme and W-CDMA (Wide band-CDMA (Code Division Multiple Access)) scheme of the CDMA schemes as access schemes in mobile communication systems share the same frequency band within the same service area.
As a mobile communication system using the CDMA scheme, a system using the IS-95 scheme is currently used in the U.S., Korea, and the like. It is, however, expected that this system will be replaced with a system used in IMT-2000 (International Mobile Telecommunications-2000) that is a multimedia mobile communication system in the process of standardization by the ITU (International Telecommunication Union).
The W-CDMA scheme is one of the schemes, the application of which to IMT-2000 has been studied. If this W-CDMA scheme is used for IMT-2000, the currently available IS-95 scheme and W-CDMA scheme may coexist. In consideration of the effective use of frequencies as well, the IS-95 scheme and the W-CDMA scheme may use the same frequency band within the same service area.
FIGS. 19 shows the arrangement of base station apparatuses in such a case.
Referring to FIG. 19, three base station apparatuses 1 are base station apparatuses based on the IS-95 scheme, and base station apparatuses 4 are base station apparatuses based on W-CDMA scheme. When a mobile station communication system based on the IS-95 scheme and a mobile station communication system based on the W-CDMA scheme are established in the same service area, base station apparatuses based on the respective schemes coexist in the area, as shown in FIG. 19.
In the communication system based on the CDMA scheme, a plurality of communication channels can use the same frequency band. This is because each communication channel is spread-modulated with codes having orthogonality on the transmission side, and each communication can be specified by spread-demodulation (despreading) with the same codes on the reception side.
This orthogonality is made imperfect by propagation delay differences due to geographical and weather conditions and the like and time deviations due to multipath in the propagation path between the mobile station and the base station, and multipath associated with irrelevant codes, i.e., irrelevant communication, and multipath associated with relevant codes, i.e., relevant communication, have correlation components in some case. These correlation components become interference components in the relevant communication, resulting in a deterioration in communication quality. Since interference components are generated by such a factor, interference components increase as the number of communication channels increases.
The number of communication channels that can commonly use the same frequency band is therefore limited. If the total transmission power exceeds a predetermined threshold in this band, predetermined communication quality cannot be obtained. In the worst case, communication failures occur.
The arrangement of the conventional base station apparatus 1 in FIG. 19 will be described next with reference to FIG. 20.
As shown in FIG. 20, the IS-95 base station apparatus 1 is comprised of a transmission/reception amplification section 10 and a modulation/demodulation section 20. The transmission/reception amplification section 10 amplifies a reception RF (Radio Frequency) signal from a mobile station and outputs the amplified signal to the modulation/demodulation section 20. The transmission/reception amplification section 10 also amplifies a transmission RF signal from the modulation/demodulation section 20 and transmits the amplified signal to each mobile station through an antenna.
The modulation/demodulation section 20 is made up of a radio section 21, a radio base station control section 22, a FACH (Forward Access CHannel) baseband signal processing section 23, an SDCCH (Stand alone Dedicated Control CHannel) baseband signal processing section 24, a TCH (Traffic CHannel) baseband signal processing section 25, and a wire transmission path interface section 26.
The radio section 21 spreads baseband signals from the FACH baseband signal processing section 23, the SDCCH baseband signal processing section 24, and the TCH baseband signal processing section 25 with spreading codes, and synthesizes the resultant signals. The radio section 21 then D/A-converts the synthetic signal, converts the analog signal into a transmission RF signal by quadrature modulation, and outputs the signal to the transmission/reception amplification section 10. In addition, the radio section 21 converts a reception RF signal from the transmission/reception amplification section 10 into an IF (Intermediate Frequency) signal, A/D-converts it, and performs quadrature demodulation of the digital signal.
The FACH baseband signal processing section 23 is made up of an encoding section 30, a decoding section 31, and a base station FACH transmission power calculation section 32. The encoding section 30 encodes a signal from the wire transmission path interface section 26 and outputs the encoded signal as a baseband signal to the radio section 21. The decoding section 31 decodes a signal demodulated by the radio section 21 and outputs the decoded signal to the wire transmission path interface section 26.
The base station FACH transmission power calculation section 32 calculates a base station FACH transmission power value from a perch CH transmission power value, a perch CH reception SIR value, a rate correction value, and FACH specified reception SIR value according to equation (1) below. The base station FACH transmission power value is a power value required to transmit a FACH signal from the base station apparatus.
base station FACH transmission power value=perch CH transmission power valuexe2x88x92(mobile station perch CH reception SIR valuexe2x88x92mobile station FACH specified reception SIR value)/FACH rate correction valuexe2x80x83xe2x80x83(1)
The FACH is a one-way channel for transmitting control information or user packet data from a base station to a mobile station. The FACH is used when the cell in which a mobile station is present is known. A FACH-L is used to transmit a relatively large amount of information, whereas a FACH-S is used to transmit a relatively small amount of information. The transmission formats for the FACH-S include a normal mode and an ACK mode. The normal mode is a mode for transmitting information about layer 3 or upper layers and packet control/user information. The ACK mode is a mode for transmitting an ACK (ACKnowledge) signal in response to a RACH (Random Access CHannel) signal received from a mobile station.
The perch CH transmission power value is a power value required to transmit a perch CH signal. This value is determined by operation information held in a base station. The perch CH is a channel for which systematic control information for each cell or sector is transmitted from a base station to a mobile station. Information whose contents change with time, e.g., SFN (System Frame Number) information and uplink interference power, is transmitted via this channel.
The perch CH reception SIR value is an SIR value obtained when a mobile station receives a perch CH signal transmitted from a base station. This information is transmitted from the mobile station to the base station via an RACH or SDCCH. The SIR (Signal Interference Ratio) is the ratio of the level of a desired signal to the level of a signal that interferes with the desired signal.
The FACH rate correction value is a value obtained by equation (2) below. This value is used to correct the influences of different transmission rates in the respective channels.
xe2x80x83FACH rate correction value=10xc3x97log (FACH transmission rate/perch CH transmission rate)xe2x80x83xe2x80x83(2)
The mobile station FACH specified reception SIR value is an SIR value to be obtained by a mobile station when it receives a FACH signal. This value is determined by operation information held in the base station.
Although not shown, the SDCCH baseband signal processing section 24 is also comprised of an encoding section, a decoding section, and a base station SDCCH transmission power calculation section.
The SDCCH is a two-way channel between a mobile station and a base station to transmit control information. This SDCCH is assigned to each mobile station for which connection control is performed. A transition from the SDCCH to an ACCH, which is an accessory control channel, after connection control operation is complete and a speech channel is established.
The encoding and decoding sections of the SDCCH baseband signal processing section 24 operate in the same manner as the encoding section 30 and decoding section 31 of the FACH baseband signal processing section 23.
The base station SDCCH transmission power calculation section of the SDCCH baseband signal processing section 24 calculates a base station SDCCH transmission power value from a perch CH transmission power value, a mobile station perch CH reception SIR value, an SDCCH rate correction value, and a mobile station SDCCH specified reception SIR value according to equation (3) below.
base station SDCCH transmission power value=perch CH transmission power valuexe2x88x92(mobile station perch CH reception SIR valuexe2x88x92mobile station SDCCH specified reception SIR value)/SDCCH rate correction valuexe2x80x83xe2x80x83(3)
The SDCCH rate correction value is a value obtained by equation (4) below. This value is used to correct the influences of different transmission rates in the respective channels.
SDCCH rate correction value=10xc3x97log (SDCCH transmission rate/perch CH transmission rate)xe2x80x83xe2x80x83(4)
The mobile station SDCCH specified reception SIR value is an SIR value to be obtained by a mobile station when it receives an SDCCH signal. This value is determined by operation information held in a base station.
Although not shown, the TCH baseband signal processing section 25 is also comprised of an encoding section, a decoding section, and TCH transmission power value calculation section. A TCH is a two-way channel between a mobile station and a base station to transmit user information.
The encoding and decoding sections of the TCH baseband signal processing section 25 operate in the same manner as the encoding section 30 and decoding section 31 of the FACH baseband signal processing section 23.
The base station TCH transmission power calculation section of the TCH baseband signal processing section 25 calculates a base station TCH transmission power value from a perch CH transmission power value, a mobile station perch CH reception SIR value, a TCH rate correction value, and a mobile station FACH specified reception SIR value according to equation (5) below.
base station TCH transmission power value=perch CH transmission power valuexe2x88x92(mobile station perch CH reception SIR valuexe2x88x92mobile station TCH specified reception SIR value)/TCH rate correction valuexe2x80x83xe2x80x83(5)
The TCH rate correction value is a value obtained by equation (6) below. This value is used to correct the influences of different transmission rates in the respective channels.
TCH rate correction value=10xc3x97log (TCH transmission rate/perch CH transmission rate)xe2x80x83xe2x80x83(6)
The mobile station TCH specified reception SIR value is an SIR value to be obtained when a mobile station receives a TCH signal. This value is determined by operation information held in the base station control apparatus.
The radio base station control section 22 controls the operation of the radio section 21 and includes an adding section 40.
The adding section 40 obtains a base station transmission power value by adding the base station FACH transmission power value obtained by the base station FACH transmission power calculation section 32, the base station SDCCH transmission power value obtained by the SDCCH transmission power value calculation section, and the base station TCH transmission power value obtained by the TCH transmission power value calculation section according to equation (7).
base station transmission power value=base station FACH transmission power value+base station SDCCH transmission power value+base station TCH transmission power valuexe2x80x83xe2x80x83(7)
The wire transmission path interface section 26 transfers a signal from each decoding section to a host unit 2 for controlling a plurality of IS-95 base station apparatuses, and transfers a signal from the host unit 2 to each encoding section.
The arrangement of the W-CDMA conventional base station apparatus 4 will be described next with reference to FIG. 21.
This conventional W-CDMA base station apparatus 4 has the same arrangement as that of the IS-95 base station apparatus 1 in FIG. 20 except that the frequency band used by the W-CDMA base station apparatus 4 is W-CDMA frequency band. A transmission/reception amplification section 60 and a modulation/demodulation section 70 in FIG. 21 therefore respectively correspond to the transmission/reception amplification section 10 and the modulation/demodulation section 20 in FIG. 20.
In addition, a radio section 71, a radio base station control section 72, a FACH baseband signal processing section 73, an SDCCH baseband signal processing section 74, a TCH baseband signal processing section 75, and a wire transmission path interface section 76 in FIG. 21 respectively correspond to the radio section 21, the radio base station control section 22, the FACH baseband signal processing section 23, the SDCCH baseband signal processing section 24, the TCH baseband signal processing section 25, and the wire transmission path interface section 26 in FIG. 20.
Furthermore, an encoding section 80, a decoding section 81, a FACH transmission power value calculation section 82, and an adding section 90 in FIG. 21 respectively correspond to the encoding section 30, the decoding section 31, the base station FACH transmission power calculation section 32, and the adding section 40 in FIG. 20.
Although not shown, similar to the FACH baseband signal processing section 73, the SDCCH baseband signal processing section 74 is comprised of an encoding section, a decoding section, and a TCH transmission power value calculation section. Similarly, although not shown, the TCH baseband signal processing section 75 is comprised of an encoding section, a decoding section, and a TCH transmission power value calculation section. The adding section 90 of the W-CDMA base station apparatus 4 obtains a base station transmission power value in a W-CDMA band.
Frequency bands in the IS-95 and W-CDMA schemes will be described next with reference to FIG. 22.
In the IS-95 scheme, the bandwidth of one communication channel is 1.25 MHz, whereas in the W-CDMA, the bandwidth of a band used to improve the multipath characteristics is as large as 5 MHz.
If, however, the sum of the base station transmission power of the IS-95 base station apparatus and the base station transmission power of the W-CDMA base station apparatus exceeds a certain threshold in one frequency band (frequency f3) in the IS-95 scheme as shown in FIG. 22, not only the communication quality of the IS-95 mobile station using the frequency band but also the communication quality of a W-CDMA mobile station deteriorate. In some case, a communication failure occurs.
In the mobile communication system using the above conventional IS-95 base station apparatuses and W-CDMA base station apparatuses, when a frequency band is shared, and the base station transmission power exceeds a threshold in one band in the IS-95 scheme, not only the communication quality of the IS-95 mobile station using the frequency band but also the communication quality of a W-CDMA mobile station deteriorate. In some case, a communication failure occurs.
It is an object of the present invention to provide an IS-95 base station apparatus, a W-CDMA base station apparatus, a mobile communication system, and a frequency sharing method, which can prevent a deterioration in communication quality when a frequency band is shared between the IS-95 scheme and the W-CDMA scheme, and the base station transmission power exceeds a threshold in a given frequency band in the IS-95 scheme.
In order to achieve the above object, according to the present invention, there is provided a frequency sharing method in an IS-95/W-CDMA scheme in which a plurality of IS-95 base station apparatuses including first base station apparatuses and a plurality of W-CDMA base station apparatuses including second base station apparatuses are arranged in the same service area, and an IS-95 scheme and a W-CDMA scheme share the same frequency bands, comprising the steps of arranging the respective IS-95 base station apparatuses and the respective W-CDMA base station apparatuses in a one-to-one correspondence, obtaining a first base station transmission power value of the first base station apparatus in each frequency band in the IS-95 scheme, obtaining a second base station transmission power value of the second base station apparatus adjacent to the first base station apparatus, determining whether there is a frequency band in the IS-95 scheme in which a sum of the first and second base station transmission power values is not less than a predetermined threshold, and when there is a frequency band in the IS-95 scheme in which the sum is not less than the predetermined threshold, performing a frequency handoff to switch a communication channel in the IS-95 scheme which is using the frequency band in which the sum is not less than the predetermined threshold to another frequency band in the IS-95 scheme.